Insulating oil



Patented Oct. 7, 1941 UNITED s'ra'rss PATENT QFF'ICE 2.251370 msomrmoon. Charles E. 'Irautman, East McKeespo r... assignor to all Research &Development Company, Pittsburgh, Pa, a corporation or Delaware.

so Drawing. Application October :1, 1940,

Serial No. 863,752 I (cusseer I This invention relates to insulatingoils; and it is particularly concerned with an improved transformer oilhaving high resistance to deterioration by oxidation and sludging,comprising a highly refined petroleum oil, a small amount of a water andalkali insoluble alkylated phenol antioxidant, particularly2,6-di-tertiarybutyl-4-methyl phenol, and a small amount ofN-tetra-hydrofurfuryi-p-aminophenol; all as more fully hereinafter setforth and as claimed.

In many types of electrical transformers the core and windings areimmersed in a suitable petroleum oil, which serves as an insulating oiland "for carrying heat away from the core and windings. Therequirements,of a transformer oil include. good dielectric properties, low viscosityto facilitate heat transfer by convection, low pour point to avoidsolidification of the oil by cold, high flash and fire points tominimize danger of fire and explosion and low volatility to reduceevaporation losses. In addition the 011 must be resistant to oxidationand sludging, and mustbe of such character as to resist emulsiflcationwith water.

Transformer oils ordinarily are distillate oils refined in such. manneras to secure the specified characteristics to as great an extent aspossible.

having four or more carbon atoms in the alkyl groups ortho to thehydroxyl group and which are substantially insoluble in water and indilute aqueous alkali, have proved particularly satisfactory asanti-oxidants in transformer oils in lengthening the induction periodbefore sludge begins to form. The compound 2,6-di-tertiarybutyl-4-methyiphenol is quite effective for this purpose. It has been found howeverthat after the induction period in a transformer oil containing acompound .of this class, the rate of sludge formation is sometimesgreater than in the original oil.

- to form 9.1 per cent of sludge in the oil is often shorter than wouldbe expected in view of the prolonged induction period.

I have discovered that this difliculty can be overcome by incorporatingin a transformer oil along with the 2,6-di-tertiary-butyl-4-methylphenol or other alkylated phenol anti-oxidant of this class, a smallamount of N-tetrahydrofurfuryl-p-aminophenol having the formula:

Achievement of satisfactory oxidation resistance is dimcult, for whileoxidational changes are slow, transformer oils are ordinarily expectedto remain in service over long periods. Oxidational changes may resultin the formation of insoluble matter or sludge. The sludge tends torender the oil more miscible with water, with production of emulsions inthe presence of condensed water and a consequent degradation of thedielectric properties of the 011 body. The sludge also deposits on thewindings and core with reduction of the emciency of heat transfer.Oxidation of transformer oils tends to be promoted by contact withmetals, especially copper, and

with the usual varnishes and insulating materials.

Various materials have been proposed for incorporation into transformeroils with the object of inhibiting oxidation, and some of these havecome into use. The usual anti-oxidants may be dividedin two principalclasseslfl) those which are associated with a lengthening of theinduction period, that is fore appearance of sludge in the oil subjectto oxidation, and (2) those which reduce the rate at which sludge isformed after it once begins to form. In evaluating oils containinganti-0x1 dants, the most useful. criterion is a value which takes intoaccount both these characteristics, viz., the time required, under thetest conditions, to form a definite amount of sludge, say 0.1 per centby weight of the oil.

The 2,4,6-tri-alkylated monohydroxy phenols the time which elapses be-2,6-dl-tertiary-butyli-methyl phenol in the usual amounts of 0.1 to 1.0per cent by weight. In general amounts ofN-tetrahydrofurfuryl-paminophenol corresponding to about 0.003 to 0.05per cent by weight have proved sufllclent although larger amounts may beusedwithout deleterious effect. The amount which may be used issubstantially limited by the saturation solubility of theN-tetrahydrofurfuryl-p aminophenol in the oil, which is usually about0.08 per cent by weight, this compound being substantially more solublein 011 than most other furfuryl-p-amino compounds.

In the following table, comparative test results on sludge formation areshown for an untreated Thus the total time required transformer .oil,the same oil containing 2,6-ditertiary-butyl-i-methyl phenol, the same01] containing N-tetrahydrofurfuryi-p-aminophenol and the same oilcontaining 2,6-di-tertlarybutyl-4-methyl phenol together withN-tetrahydrofurfuryl-p-aminophenol. The test data. were obtained inaccordance with the sludge formation test for mineral transformer oil,described by F. M. Clark and E. A. Snyder in their paper entitledTesting for Sludge Formation in Mineral Transformer Oil" presented atthe thirty-ninth annual meeting for the American Society for TestingMaterials. The test procedure described in the aforesaid paper wasdeveloped in collaboration with the laboratories of a considerablenumber of firms engaged in oil refining and in the manufacture ofelectrical equipment.

In this test, samples of oil are maintained in tubes containing copperwire at a constant temperature of 120 C. 0.5 C. The tubes are open atthe top to an enclosed air space and dry air is passed at a controlledrate of 1% to 2 cubic feet per hour through the enclosed space above thesame tubes. Into each tube is introduced 22 grams of oil or oilcomposition to-be tested, a number of tubes being employed. Atprescribed intervals the tubes are successively removed and the amountof sludge determined. The results of these tests are reported in termsof the induction period, the percent of sludge formed per day and thetotal number of days taken to form 0.1 per cent of sludge.

The oil employed in all of the tests set forth hereinbelow was a typicalpetroleum transformer oil, commonly used for the purpose indicated.

(A) -0.37 2 B-di-tertiary-butyl-i-methyl phenol. (B) 0.02%.,N-tetmhydroiuriuryi-p-aminophenol.

It will be observed from these results that while the rate of sludgeformation after the induction period was substantially increased by2,6-di-tertiary-butyl i-methyl phenol and was substantially decreased byN-tetra-hydrofurfuryl-p-aminophenol the two reagents together have arate lower even than that for the N-tetrahydrofurfuryl-p-aminophenolalone; and while the N-tetrahydrofurfuryl-p-aminophenol decreased theinduction period of the oil containing 2,6-di-tertiary-butyl-4-methylphenol the net result of the combined action of the 2,6,-di-tertiary-butyl-i-methyl phenol and the N-tetrahydroiuriuryl-p-aminophenol was to give an oil having a greatlyprolonged useful life.

While my invention has been described herein with particular referenceto transformer oil compositions, the combination of2,6-di-tertiarybutyl4-methyl phenol withN-tetrahydrofurfuryl-p-aminophenol is equally eilective in stabilizingother forms of electrical insulating oils against oxidationaldeterioration, and insulating oils generally containing stabilizingamount of these compounds are included within the scope of my invention.

What I claim is:

1. An electrical insulating oil composition comprising a petroleum oilhaving a stabilizing amount of an alkali insoluble 2,4,6-tri-alkylated'mono-hydroxy phenol and a stabilizing amount ofN-tetrahydrofurfuryl-p-aminophenol dissolved therein.

2. An electrical petroleum oil having 0.1 to 1.0 per cent by weight of2,6-di-tertiary-butyl-4-methy1 phenol and 0.003 to 0.08 per cent byweight of N-tetrahydrofurfuryl-p-aminophenol dissolved therein.

CHARLES E. TRAUTMAN.

insulating oil composition comprising a petroleum oil having a smal

